Streamlined Subglacial Bedform Sensitivity to Bed Characteristics Across the Deglaciated Northern Hemisphere

Streamlined subglacial bedforms observed in deglaciated landscapes provide the opportunity to assess the sensitivity of glacier dynamics to bed characteristics across broader spatiotemporal scales than is possible for contemporary glacial systems. While many studies of streamlined subglacial bedforms rely on manual mapping and qualitative (i.e., visual) assessment, we semi-automatically identify 11,628 sedimentary and bedrock bedforms, created during and following the Last Glacial Maximum across nine geologically and topographically diverse deglaciated sites in the Northern Hemisphere. Using this large dataset of landforms and associated morphometrics, we empirically test the importance of subglacial terrain on bedform morphology and ice-flow behavior. A minimum bedform length–width ratio threshold provides a constraint on minimum morphometrics needed for streamlined bedforms to develop. Similarities in bedform metric distribution regardless of bed properties indicate that all bed types may support similar distributions of warm-based ice flow conditions. Ice flow within valleys with easily erodible beds host the most elongate bedforms yet the widest range in bedform elongation and bedform surface relief. The presence of these highly elongate bedforms suggest high ice-flow velocities occur within valley settings despite spatially heterogeneous landform-generating processes. In contrast, lithified sedimentary beds within regions not constrained by topography on the scale of 1–102 km contain bedforms with high density and packing, low change in surface relief and low elongation, indicating spatially uniform and organized interactions at the ice–bed interface and consistency in ice-flow velocity. Regardless of genesis, we find a sensitivity of bedform elongation (i.e., used to interpret ice-flow speed or persistence) to topographic conditions on the scale of 1–102 km, while bedform density is sensitive to bed lithology. The findings presented in this study provide analogues for processes of subglacial erosion and deposition, ice–bed interactions and warm-based ice flow within contemporary glacial systems

Advances in understanding subglacial meltwater drainage from past ice sheets

Meltwater drainage beneath ice sheets is a fundamental consideration for understanding ice–bed conditions and bed-modulated ice flow, with potential impacts on terminus behavior and iceshelf mass balance. While contemporary observations reveal the presence of basal water movement in the subglacial environment and inferred styles of drainage, the geological record of former ice sheets, including sediments and landforms on land and the seafloor, aids in understanding the spatiotemporal evolution of efficient and inefficient drainage systems and their impact on ice-sheet behavior. We highlight the past decade of advances in geological studies that focus on providing process-based information on subglacial hydrology of ice sheets, how these studies inform theory, numerical models and contemporary observations, and address the needs for future research

Advances in understanding subglacial meltwater drainage from past ice sheets

Meltwater drainage beneath ice sheets is a fundamental consideration for understanding ice–bed conditions and bed-modulated ice flow, with potential impacts on terminus behavior and ice-shelf mass balance. While contemporary observations reveal the presence of basal water movement in the subglacial environment and inferred styles of drainage, the geological record of former ice sheets, including sediments and landforms on land and the seafloor, aids in understanding the spatiotemporal evolution of efficient and inefficient drainage systems and their impact on ice-sheet behavior. We highlight the past decade of advances in geological studies that focus on providing process-based information on subglacial hydrology of ice sheets, how these studies inform theory, numerical models and contemporary observations, and address the needs for future research

Rapid retreat of Thwaites Glacier in the pre-satellite era

Understanding the recent history of Thwaites Glacier, and the processes controlling its ongoing retreat, is key to projecting Antarctic contributions to future sea-level rise. Of particular concern is how the glacier grounding zone might evolve over coming decades where it is stabilized by sea-floor bathymetric highs. Here we use geophysical data from an autonomous underwater vehicle deployed at the Thwaites Glacier ice front, to document the ocean-floor imprint of past retreat from a sea-bed promontory. We show patterns of back-stepping sedimentary ridges formed daily by a mechanism of tidal lifting and settling at the grounding line at a time when Thwaites Glacier was more advanced than it is today. Over a duration of 5.5 months, Thwaites grounding zone retreated at a rate of >2.1 km per year—twice the rate observed by satellite at the fastest retreating part of the grounding zone between 2011 and 2019. Our results suggest that sustained pulses of rapid retreat have occurred at Thwaites Glacier in the past two centuries. Similar rapid retreat pulses are likely to occur in the near future when the grounding zone migrates back off stabilizing high points on the sea floor

Overdose Prevention and Naloxone Prescription for Opioid Users in San Francisco

Opiate overdose is a significant cause of mortality among injection drug users (IDUs) in the United States (US). Opiate overdose can be reversed by administering naloxone, an opiate antagonist. Among IDUs, prevalence of witnessing overdose events is high, and the provision of take-home naloxone to IDUs can be an important intervention to reduce the number of overdose fatalities. The Drug Overdose Prevention and Education (DOPE) Project was the first naloxone prescription program (NPP) established in partnership with a county health department (San Francisco Department of Public Health), and is one of the longest running NPPs in the USA. From September 2003 to December 2009, 1,942 individuals were trained and prescribed naloxone through the DOPE Project, of whom 24% returned to receive a naloxone refill, and 11% reported using naloxone during an overdose event. Of 399 overdose events where naloxone was used, participants reported that 89% were reversed. In addition, 83% of participants who reported overdose reversal attributed the reversal to their administration of naloxone, and fewer than 1% reported serious adverse effects. Findings from the DOPE Project add to a growing body of research that suggests that IDUs at high risk of witnessing overdose events are willing to be trained on overdose response strategies and use take-home naloxone during overdose events to prevent deaths

Streamlined subglacial bedforms across the deglaciated Northern Hemisphere

These data include shapefiles of streamlined subglacial bedforms, elongate bedforms created at the glacier-bed interface by erosional and depositional processes, characterized by polygons across nine deglaciated geographic locations in the Northern Hemisphere. Assessed deglaciated sites include the Puget Lowland, Washington, United States; northwestern Pennsylvania, United States; Chautauqua, New York, United States; M'Clintock Channel, Canada; Prince of Wales Island, Canada; Nunavut, Canada; Bárðardalur, Iceland; northern Norway; and northern Sweden. These data are based on digital elevation models (DEMs) published by Clallam County, 2005, Porter et al., 2018, and the United States Geologic Survey, 1999, 2000. Streamlined subglacial bedform polygons were identified semi-automatically utilizing Topographic Position Index (TPI) methodology, used to calculate elevation and slope variations across any Digital Elevation Model (Weiss, 2001; Tagil & Jenness, 2008), and manual assessment of the landscape. The TPI ArcPython code and ArcGIS model builder tool developed and utilized in this project are also included in the attached data. Shapefile attribute tables and excel file include streamlined subglacial bedform morphologies for all 11,628 bedforms including area, width, length, orientation, elevation and slope ranges, and elongation. The morphological characteristics of these streamlined subglacial bedforms provide information on ice-streaming characteristics across variable bed lithology and topographic settings

Diagnosing ice sheet grounding line stability from landform morphology

The resilience of a marine-based ice sheet is strongly governed by the stability of its grounding lines, which are in turn sensitive to ocean-induced melting, calving, and flotation of the ice margin. Since the grounding line is also a sedimentary environment, the constructional landforms that are built here may reflect elements of the processes governing this dynamic and potentially vulnerable environment. Here we analyse a large dataset (n=6275) of grounding line landforms mapped on the western Ross Sea continental shelf from high-resolution geophysical data. The population is divided into two distinct morphotypes by their morphological properties: recessional moraines (consistently narrow, closely spaced, low amplitude, symmetric, and straight) and grounding zone wedges (broad, widely spaced, higher amplitude, asymmetric, sinuous, and highly variable). Landform morphotypes cluster with alike forms that transition abruptly between morphotypes both spatially and within a retreat sequence. Their form and distribution are largely independent of water depth, bed slope, and position relative to glacial troughs. Similarly, we find no conclusive evidence for morphology being determined by the presence or absence of an ice shelf. Instead, grounding zone wedge construction is favoured by a higher sediment flux and a longer-held grounding position. We propose two endmember modes of grounding line retreat: (1) an irregular mode, characterised by grounding zone wedges with longer standstills and accompanied by larger-magnitude retreat events, and (2) a steady mode, characterised by moraine sequences that instead represent more frequent but smaller-magnitude retreat events. We suggest that while sediment accumulation and progradation may prolong the stability of a grounding line position, progressive development of sinuosity in the grounding line due to spatially variable sediment delivery likely destabilises the grounding position by enhanced ablation, triggering large-magnitude retreat events. Here, the concept of “stability” is multifaceted and paradoxical, and neither mode can be characterised as marking fast or slow retreat. Diagnosing grounding line stability based on landform products should be considered for a wider geographic range, yet this large dataset of landforms prompts the need to better understand the sensitivity of marine-based grounding lines to processes and feedbacks governing retreat and what stability means in the context of future grounding line behaviour

Exceptions to bed-controlled ice sheet flow and retreat from glaciated continental margins worldwide

Projections of ice sheet behavior hinge on how ice flow velocity evolves and the extent to which marine-based grounding lines are stable. Ice flow and grounding line retreat are variably governed by the coupling between the ice and underlying terrain. We ask to what degree catchment-scale bed characteristics determine ice flow and retreat, drawing on paleo-ice sheet landform imprints from 99 sites on continental shelves worldwide. We find that topographic setting has broadly steered ice flow and that the bed slope favors particular styles of retreat. However, we find exceptions to accepted “rules” of behavior: Regional topographic highs are not always an impediment to fast ice flow, retreat may proceed in a controlled, steady manner on reverse slopes and, unexpectedly, the occurrence of ice streaming is not favored on a particular geological substrate. Furthermore, once grounding line retreat is under way, readvance is rarely observed regardless of regional bed characteristics

Streamlined subglacial bedforms across isolated topographic highs in the Puget Lowland, Washington state

These data include shapefiles of streamlined subglacial bedforms, elongate bedforms created at the glacier-bed interface by erosional and depositional processes, characterized by polygons across nine isolated bedrock highs in the deglaciated Puget Lowland of Washington state. These data are based on digital elevation models (DEMs) published by Clallam County, Washington Department of Natural Resources, WA, 2008, Quantum Spatial Inc., 2017, 2019; OCM Partners, 2019a, 2019b. Streamlined subglacial bedforms polygons were identified using two semi-automated tools utilizing Topographic Position Index (TPI) methodology (McKenzie et al., 2022) and contour tree-mapping (Wang et al., 2017) used to calculate slope and elevation variations across any Digital Elevation Model, respectively. Shapefile attribute tables and excel file include streamlined bedform morphologies for all 3,273 bedforms categorized into upstream, on top of, or downstream of the topographic bump based on the direction of ice flow. The morphometric information for all bedforms includes features area, width, length, orientation, elevation and slope ranges, and elongation. The morphological characteristics of these streamlined subglacial bedforms provide information on ice-streaming characteristics and changes across topographic highs in the subglacial environment